Online video ads are one of the fastest-growing ad mediums, far outpacing growth in spending on television and other digital formats. Online video ad viewing exploded in 2013. Over 35 billion video ads were viewed in the U.S. in December.

It's not hard to understand what makes online video so compelling to advertisers. Video ads provide a level of visual and narrative richness that nearly equals television, while offering all the advantages of digital, including advanced targeting, tracking, and increasingly, automated buying of video ad units.